rtc: pcf8523: fix for stop bit

Bugfix for an issue detected when a goldcap capacitor gets
fully discharged due to a long absence of the power supply,
and then recharges again. The RTC failed to continue to keep
the real-time clock.

This was caused by the incorrect handling of the STOP bit in
the RTC internal register.  This fix solves the problem.

Signed-off-by: paulmn <paulmn@axis.com>
Link: https://lore.kernel.org/r/20220829124639.10906-1-paulmn@axis.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>